1. Field of the Invention
The present invention relates to a pretensioner for a webbing retractor which removes the slackness in a webbing by forcefully winding the webbing onto a winding shaft when a vehicle executes emergency deceleration in a collision or the like.
2. Description of the Related Art
A webbing retractor is provided with a pretensioner for removing the slackness in a webbing by forcefully winding the webbing onto a winding shaft when a vehicle executes emergency deceleration in a collision or the like.
This type of pretensioner includes what is known as a rack and pinion type of pretensioner which is provided in the webbing retractor 50 shown in FIG. 5. This webbing retractor 50 is provided with a pinion 56 which is connected to a winding shaft 60 onto which the webbing 58 is wound so as to always rotate integrally with the winding shaft 60. A cylinder 52 is provided near the pinion 56 so that when the vehicle executes an emergency deceleration, gas is supplied to the inside of the cylinder 52.
A piston 62 is provided so as to be movable inside the cylinder 52. When the vehicle executes an emergency deceleration, the piston 62 is moved by the pressure of the gas in the direction of the arrow C shown in FIG. 5. A rack 64 is provided integrally with the piston 62 on the surface thereof nearest the pinion 56. The rack 64 is provided so as to move together with the piston 62 thereby meshing with the pinion 56 and causing the pinion 56 to rotate in the direction in which the webbing 58 is retracted (the direction shown by the arrow A in FIG. 5). Accordingly, in an emergency deceleration of the vehicle, the webbing 58 is wound onto the winding shaft 60 and the slackness in the webbing 58 is removed.
The piston 62 is positioned in its initial position by abutting the bottom portion 66 of the cylinder 52. A seal holding groove 68 is formed in the side surface of the piston 62 by cutting the side surface of the piston 62 and an elastic O ring 70 is mounted in the seal holding groove 68. The O ring 70 seals the gap between the piston 62 and the inner wall 72 of the cylinder 52 thereby increasing the air-tightness between the piston 62 and the cylinder 52.
It should be noted that in this type of webbing retractor 50, costs are increased because of the labor required by the cutting process for forming the seal holding groove 68 and stretching the O ring 70 to fit into the seal holding groove 68 is difficult.
Because of this, in, for example, the webbing retractor 80 described in Japanese Patent Application Laid Open (JP-A) 9-202213 (refer to FIG. 6), the piston 82 has a structure where a piston body 84 and a pressure receiving plate 86 are formed separately and an O ring 88 is sandwiched between the piston body 84 and the pressure receiving plate 86. Because the O ring 88 is sandwiched between the piston body 84 and the pressure receiving plate 86, this webbing retractor 80 does not require the cutting process to form the seal holding groove 68 or the task of fitting the O ring 70 into the seal holding groove 68.
However, because the piston 82 of the webbing retractor 80 has a structure where a piston body 84 and a pressure receiving plate 86 are formed separately, the number of parts is increased thereby leading to increased costs. Moreover, there is a drawback in that a noise from the contact is generated when the bottom portion 66 of the cylinder 52 comes into contact with the pressure receiving plate 86 of the piston 82 due to vibration and the like.
Moreover, if the webbing retractor 50 shown in FIG. 5 is provided with what is known as a force limiter mechanism to prevent the load on a vehicle occupant from rising above a predetermined value when the vehicle executes an emergency deceleration, then when the force limiter operates, the pinion 56 rotates in the opposite direction to the direction in which the webbing 58 is retracted (the direction shown by the arrow B in FIG. 5) thus causing the piston 62 to move towards the bottom portion 66 of the cylinder 52 (the direction shown by the arrow D in FIG. 5) and to abut the bottom portion 66 of the cylinder 52. Therefore the pinion 56 and the rack 64 lock up and are damaged. Moreover, because the piston 62 cannot move below its initial position, there is a possibility of the operation of the force limiter being affected.